Earlier, U.S. officials told NBC News they were investigating terrorism concerns after the red-eye flight from Kuala Lumpur, Malaysia, to Beijing vanished in relatively clear weather, without sending a distress signal, at what analysts said would have been cruising altitude.

"We are aware of the reporting on the two stolen passports," a senior U.S. official said. "We have not determined a nexus to terrorism yet, although it's still very early, and that's by no means definitive."

Malaysian Transport Minister Hishammuddin Hussein said help was also being sought from the FBI.

"We are looking at all possibilities," he added. "We cannot jump the gun. Our focus now is to find the plane."

A Malaysian official with knowledge of the investigation told Reuters the passengers who used the stolen passports had bought their tickets through China Southern Airlines, which was code-sharing the flight with Malaysia Airlines.

European authorities confirmed the stolen Italian-issued document was registered to Luigi Maraldi and the Austrian passport was under the name Christian Kozel.

A telephone operator on a China-based KLM hotline Sunday confirmed to The Associated Press that "Maraldi" and "Kozel" were both booked to leave Beijing on a KLM flight to Amsterdam on Saturday. Maraldi was then to fly to Copenhagen on KLM and Kozel to Frankfurt.

Maraldi's father, Walter, told NBC News on Saturday that his 37-year-old son was vacationing in Thailand, adding that his passport had been stolen in that country a year ago.

Meanwhile, Vietnamese naval boats sent from the holiday island of Phu Quoc patrolled stretches of the Gulf of Thailand, scouring the area where an oil slick was spotted by patrol jets just before nightfall Saturday.

"Our two rescue boats have approached the two oil spills since 3 a.m. today, but we haven't found any sign of the Malaysian plane yet. Other boats are ready to go to support if needed," Adm. Ngo Van Phat told Reuters.

The search was being extended to the west coast of the Malay peninsula, in addition to a broad expanse of the sea between Malaysia and Vietnam, Malaysian air force chief Rodzali Daud said.

Besides the Vietnamese vessels, Malaysia and neighboring countries have deployed 22 aircraft and 40 ships in the search. China and the U.S. have sent ships to help, and Washington has also deployed a plane.

The 11-year-old Boeing 777-200ER, powered by Rolls-Royce Trent engines, took off at 12:40 a.m. (11:40 a.m. ET Friday) from Kuala Lumpur International Airport, then went missing without a distress call.

It last had contact with air traffic controllers 120 nautical miles off the east coast of the Malaysian town of Kota Bharu. Flight tracking website FlightAware.com showed that it flew northeast after takeoff, climbed to 35,000 feet and was still climbing when it vanished from tracking records.

Air force chief Rodzali didn't say which direction the plane might have taken or for how long when it apparently went off route. "We are trying to make sense of this," he said at a news conference. "The military radar indicated that the aircraft may have made a turn back, and in some parts, this was corroborated by civilian radar."